Lesson 3: Decimals (Group Lesson)

Add, Subtract, Multiply & Divide

28.9 + 32.5

61.4

Period 3, 5: 9/17/15Period 2, 4, 6: 9/18/15

Group Lesson

Lesson Objective

SWBAT …1) Add numbers with decimals.2) Subtract numbers with decimals. 3) Multiply numbers with decimals. 4) Divide numbers with decimals.

Adding or Subtracting Decimals

Key Points (review, don’t write down)

Write the numbers in a vertical column, aligning digits according to their places.

Attach extra zeros to the right end of each number so each number has the same amount of digits.

Add or subtract as though the numbers are whole numbers.

Place the decimal point in the sum or difference to align with the decimal point in the respective operation.

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Adding Decimals

Add the ones. There are 18 ones.

Add the tenths

8.7842.560

+ 6.721

Ones

Tenth

s

Thousa

ndth

s

Add the hundredths.

12

0

Add up the thousandths.

So 8.784 + 2.56 + 6.721 = 18.065.NM Spirit

Subtracting Decimals

Subtract the thousandths.

4.040 -2.6811.359

So, 4.040 – 2.681 = 1.359

Subtract the hundredths. There are 5 hundredths.

Subtract the tenths. There are 3 tenths.

Subtract the ones. There is 1 one.

9 13 10

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Multiply Decimals

Key Points (review, don’t write down):

Multiply the decimal numbers as though they are whole numbers.

Count the digits in the decimal parts of both decimal numbers.

Place the decimal point in the product so that there are as many digits in its decimal part as there are digits you counted in the previous step.

If necessary, attach zeros to the left end of the product to place the decimal point accurately.

21.4x 0.36

3 total decimal places

7 704.NM Spirit

Look at this example.3.45 x 4.082 =

How many places are there to the right of the decimal point?

Five; so, the answer will have five places to the right of the decimal.

The answer is 14.08290

The last zero can be dropped and the answer would be 14.0829.

Examples 920 x 3.7

= 3404.00.00079 x 0.025

= 0.00001975

Class Examples870 x 4.6

= 4002.00.000086 x 0.057

= 0.000004902NM Spirit

Dividing decimals is similar to dividing whole numbers. Same question…what about the decimal place?

Where does that go?

Steps

1. Make the divisor a whole number by shifting the decimal to the right as many times as necessary.

2. Move the decimal in the dividend the same number of times that we moved it in the divisor

7 0 6 4 2 0 9. . 0 ??????

Division of Decimals

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• Dividing decimals……Steps:1. Add zeros to the end of the dividend so that

we can round to the desired place value– Example: Round quotient to nearest tenth write 2

zeros after the decimal

– Round quotient to nearest thousandth need 4 zeros after the decimal

706 42090.00

706 42090.0000

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Dividing decimals……. Steps1. Do the division as if it were whole numbers2. Put the decimal place in the quotient

directly over the decimal point in the dividend

706 42090.00

00059.61 ≈ 59.6

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Each of the digits are the same. Each of the steps are the same. But the place values are different because of the decimal points.

3751814

7.38.5114

-570 76 -76 0

-57 0 7 6 -7 6 0

Examples #

0.378.514.1

-57 0 7 6 -7 6 0

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Whiteboard CFUQuiz Your Group

Evaluate the expression:

43-27.4

Whiteboard CFUQuiz Your Group

45.56+ 23 + 4.5

Whiteboard CFUQuiz Your Group

9-0.5+ 4.67

Whiteboard CFUQuiz Your Group

15.5 X 26.77